Tolerance to a new herbicide, pyributycarb, was evaluated both at the plant and cellular levels. Several highly or moderately tolerant strains chosen at the plant level, showed a parallel relation of to tolerance at the cellular level. However, on the whole, correlation between total tolerance indices and survival rates of calli was not significant in 18 out of the 80 studied strains. As a result of somaclonal selection for two herbicides, lines NB-200 and NM-100 were regenerated from the tolerant calli screened with benthiocarb at 200 ppm, and molinate at 100 ppm, respectively. In the R2 generation, both the lines displaned a stable tolerance both at the plant and cellular levels. Thus the highly tolerant mutant lines were developed from a moderately tolerant line, N-61, via in vitro selection. To achieve a short-cut method in the interspecific genetic exchange, a series of techniques related to cell fusion were established in rice and related species. Two kinds of somatic hybrids between the cultivar Kitaake and tetraploid Oryza species, O. punctata and O. officinalis, were successfully produced. Among the somatic hybrid plants, a wide range of chromosomal variation was observed. Aneuploid plants with a chromosome number around 2n = 72 (hexaploid), which are expected from a symmetric fusion between diploid and tetraploid strains, were obtained showing mixoploidy within a plant. Most of the somatic hybrids were characterized by intermediate features of plant-type showing high sterility, shattering of spikelets and reduced plant height. As an exception, a diploid plant, which was identified by RFLP analysis using the rDNA gene probe, closely resembled Kitaake and produced viable seeds. A tetraploid hybrid plant was also promising for the introduction of economically important characters through the reduction of chromosome numbers by doubled haploids. Gametoclonal variation and gamma radiation was applied to Kitaake. The mutation frequency was prominently increased by gamma ray treatment, especially at high doses of 200 Gy or 300 Gy. In the M3R2 or M4R3 generations, most of the variants showed unfavourable characters. Most of the mutant characters were governed by single or double recessive genes. Several mutants such as short culm and early flowering time might be used for rice breeding.